A circular celebration © Lingxiao Xie/Getty Images

It"s Pi Day, the day we celebrate the mathematical constant π. This day was chosen because today"s date, March 14, expressed numerically, matches the number"s first digits, 3.14. Pi represents the ratio of a circle"s circumference to its diameter. It can be used to compute the area and circumference of a circle, such as the circles that make up this bridge we"re looking at here in Copenhagen. (Who said math wasn"t useful in the real world?) Many people mark the day with activities based around mathematics—and most also serve pie. We can"t think of a more delicious way to study geometry.